Migrate `JavaScriptContextHolder` & `JavaScriptExecutor` to Kotlin (#49909)

Summary:
Migrate com.facebook.react.bridge JavaScriptContextHolder & JavaScriptExecutor to Kotlin.

In this PR I also marked the return of HermesExecutor.initHybridDefaultConfig as non-nullable to keep the conversion without logic changes. I checked that this was changed unintentionally to nullable in 12e321daf07d312c6b4947e306a449a7985ac19a but I think it was not supposed to be.

## Changelog:

[INTERNAL] - Migrate com.facebook.react.bridge JavaScriptContextHolder & JavaScriptExecutor to Kotlin

Pull Request resolved: https://github.com/facebook/react-native/pull/49909

Test Plan:
```bash
yarn test-android
yarn android
```

Reviewed By: javache

Differential Revision: D70871012

Pulled By: Abbondanzo

fbshipit-source-id: 1c15927056baf166e520e896e92bebf664fa0229